1. Field of the Invention
The present invention relates to a new process for the preparation of polyurea elastomers containing urethane groups from polyisocyanates of the diphenylmethane series, high molecular weight polyhydroxy compounds, alkyl-substituted aromatic diamines and high molecular weight polyamino compounds in accordance with reaction injection molding process.
2. Description of the Prior Art
The preparation of polyurea elastomers containing urethane groups in accordance with the reaction injection molding (RIM) process using polyisocyanates or polyisocyanate mixtures of the diphenylmethane series, high molecular weight polyether polyols and alkyl-substituted aromatic diamines is known and disclosed DE-AS 2 622 951 (U.S. Pat. No. 4 218 543). The elastomers are prepared in accordance with the one-shot process, i.e., the polyisocyanate component is reacted with a mixture of the compounds having isocyanate-reactive groups. The process also uses catalysts for the reaction between isocyanate groups and hydroxyl groups.
U.S. Pat. No. 4 374 210 relates to a modification of this process wherein 0.01 to 0.5 equivalent of the high molecular weight polyol component is initially reacted with the polyisocyanate component so that reaction streams of approximately equal volume can be reacted with each other during subsequent processing. In fact, the authors of U.S. Pat. No. 4 374 210 believed to have recognized a disadvantage of the process according to DE-AS 2 622 951, i.e., that significantly uneven flow volumes (low volume of the polyisocyanate component and high volume of the isocyanate-reactive component) are mixed and reacted with each other according to this process.
Surprisingly, it has now been found that an essentially complete preliminary reaction of the high molecular weight polyol component with the polyisocyanate component and subsequent reaction of the resulting NCO semi-prepolymer with the amine component leads to a number of remarkable advantages, although extremely different volume flows have to be reacted with one another in this process (high volume of the NCO semi-prepolymer and low volume of the amine component).
The reaction of pure or polymeric 4,4'-diisocyanatodiphenylmethanes having NCO contents of 30-33.6 wt. % with alkyl-substituted aromatic diamines and polyol mixtures is not possible, since the reactivity of these isocyanates is too high. Liquid semi-prepolymers having NCO contents which vary between 18 and 27 wt. % are therefore initially prepared by a preliminary reaction with aliphatic polyhydroxy compounds having a molecular weight of 62 to 700, such as ethylene glycol or tripropylene glycol. These semi-prepolymers are then reacted to form elastomeric moldings by the RIM process, as described, e.g., in DE-AS 2 622 951. The disadvantage of this process is that the urethane groups formed in the preliminary reaction have low thermal stability and therefore the maximum heat distortion temperature, based on the number of reacting NCO groups, cannot be achieved. A further disadvantage of this process is the use of "high-quality" compounds, i.e., expensive isocyanates, for producing the semi-prepolymers, which do not result in the optimum elastomer properties.
The process according to the invention described in more detail below has, in particular, the following advantages:
1. The greatest proportion of the isocyanate groups is used for building up urea hard segments, and only a minimum of urethane groups is required. This results in a significant savings in the expensive isocyanate in comparison with the one-shot RIM process.
2. For the same amounts of diamines and polyisocyanates, a significant improvement is obtained in the mechanical and thermomechanical properties, in particular the heat distortion temperature, and also low temperature properties.
The torsion curve shows a sharp drop in the low temperature range (-40.degree. to O.degree. C.) and a flatter course in the range above 100.degree. C. in comparison with the curves of elastomers prepared by the one-shot process.
3. The use of catalysts which have a marked adverse influence on the heat distortion temperature and the resistance to aging can be dispensed with completely, since the OH groups of the polyhydroxy compounds, which react slowly in comparison with the reactive amino groups, have already reacted in the preliminary reaction.
U.S. Pat. No. 4 297 444 describes the production of polyurethane shaped articles in accordance with reaction injection molding process using organic polyisocyanates, high molecular weight polyhydroxy compounds and low molecular weight chain lengthening agents. In accordance with a preferred embodiment of this process the total amount of the high molecular weight polyhydroxyl component is initially reacted with the polyisocyanate component to produce an NCO semi-prepolymer. However, this prior publication gives no indication that systems of the type to be employed according to the present invention, i.e., those in which the diamine component is based on alkyl-substituted aromatic amines, would also be processable in accordance with this principle, nor does this prior publication recognize the previously described advantages of this process. Although the prior publication also discloses aromatic diamines as suitable chain lengthening agents, it does indicate that aromatic diamines are preferred. Rather, preferred chain lengthening agents are identified as low molecular weight diols such as ethylene glycol or 1,4-butanediol. Ethylene glycol is exclusively used in the examples. Therefore, the processability of reactive systems based on a large flow volume of NCO semi-prepolymer and a small flow volume of highly reactive polyamines by the reaction injection molding process in accordance with the present invention could not be predicted based on the disclosure of U.S. Pat. No. 4 297 444.
The process according to the invention also could not be predicted from the teachings of DE-AS 1 240 654. In this process, as can be seen from the examples, monomer-free, i.e., true, NCO prepolymers based on polyisocyanates and difunctional polyhydroxy compounds are preferably reacted, more preferably in the presence of solvents, with alkyl-substituted diamines in accordance with the classical prepolymer process and not in accordance with the reaction injection molding process.